Environmental Engineering Reference
In-Depth Information
To sor t out t h is paradox, successional attributes were compared in landscapes in
northern Maine, USA, where lynx were present or absent according to surveys of
tracks in the snow (Hoving et al., 2004). High hare densities were found to be asso-
ciated with densely regenerating forest following complete removal (clear cutting)
of trees. Lynx are most likely to occur in landscape units (each 100 km
2
) that contain
a large proportion of regenerating forest, least likely to occur where there has been
recent clear-cut or partial forest felling, and are neither positively nor negatively
associated with mature forest. And snowshoe hares showed precisely the same
pattern.
Clear cutting is benefi cial to lynx in the long term (but not for the fi rst several
years) because it produces dense forest regeneration with abundant snowshoe hares.
This pattern of forest harvest may well mimic large-scale natural disturbances such
as fi re or occasional insect outbreaks that kill trees over large areas. On the other
hand, a recent trend towards partial harvesting, of the type that maintains forest
composition at a relatively stable state (Section 7.3.2), seems to favor neither hares
nor lynx and may actually jeopardize the threatened lynx population. Forest har-
vesting will need to be re-planned so that suitably sized patches of regenerating
forest (from clear-cut areas) are available in the landscape.
8.5.2
Enforcing a
successional mosaic
- fi rst aid for
butterfl ies
Just as lynx need snowshoe hares, butterfl ies need suitable host plants for their cat-
erpillars to feed on. Things are not as they once were in piñon-juniper woodland
(
Pinus edulis
,
Juniperus monosperma
), the most common vegetation in New Mexico
and Arizona, USA. During the last century or so livestock grazing and periodic
drought have reduced herbaceous ground cover, interrupting the fi re regime and
leading to dense forest that lacks the original more open mosaic patches with grassy
understories and rich soils. Kleintjes et al. (2004) assessed a management regime
called overstory reduction and slash mulching (ORSM) to restore the former habitat
mosaic, with expected benefi ts for host plants and their butterfl ies.
ORSM involves removing small trees (
20 cm diameter at their base) and main
branches of larger trees, applying the resulting slash as a fi ne surface layer of organic
matter (mulch), particularly to areas of eroded soil. Four years after this treatment
both abundance and species richness of butterfl ies were signifi cantly higher than in
untreated woodland. The open patches in the restored habitat mosaic had greater
ground cover of grasses and forbs, including fi ve of the ten most common caterpillar
<
Fig. 8.16
(a) Abun-
dance (±95% confi dence
limits) and (b) species
richness of butterfl ies
are increased 4 years
after overstory
reduction and slash
mulching (ORSM =
'Treated', in comparison
to 'Control') to create
open habitat patches in
piñon-juniper woodland
in New Mexico. The
surveys were carried
out on two occasions in
mid-summer. (After
Kleintjes et al., 2004.)
(a)
(b)
30
25
20
15
10
5
0
30
25
20
15
10
5
0
Control
Treated
14 -15 June
2001
13 -16 July
2001
Sample dates
14 -15 June
2001
13 -16 July
2001
Sample dates
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